Wiring 3rd Brake Light on Truck Cap for 2015 GMC Sierra 2500 Duramax

Question:

I have a 2015 gmc 2500 duramax, just put a truck cap on and need to wire in the 3rd brake light. I’ve talked to several diesel mechanics and they all say the same thing about having a blue wire at the spare tire in the trailer harness cluster. There is a blue wire but it is connected to the 7-pin harness. Is this the wire I should be tying into or should I be looking for another wire that is capped further back? From what I found in the wire cluster, I only have one blue wire going up over the spare tire. It’s a simple 2-wire connection to the cap but I need clarification before I tie into the wrong wire or rip out the entire wire bundle. Thanks for your help.

asked by: Aaron D

Expert Reply:

The dark blue wire that you found actually coming from your Sierra 2500's 7-Way connector carries the signal for the trailer's electric brakes. You won't want to tap into this wire for the third brake light on your topper because of the variance in voltage that would be carried on this wire that is dependent on your braking. This would result in a brake light on your topper that can be extremely dim or bright depending on how hard you're slamming on the brakes or how you have the brake controller adjusted.

There may be a separate lighter blue wire further up the harness by the spare tire like you mentioned that is used for third brake light applications but from what I could find it looks like this was present on earlier models of your Sierra. If you do find another light blue wire, I recommend testing it with a circuit tester like # PTW2993 to make sure it is only powered when the brake lights are activated on the truck.

If you can't find the wire in question your next best option is to tap into the turn/stop light signals behind the taillights. You can use Pacer 3rd Brake Light Logic Module # PP20-702 to wire them to the light on your topper. The module features a built-in converter that will filter out the turn signal inputs (so the brake light doesn't also illuminate with your turn signals).

I have attached a diagram so you can see exactly how it wires.

expert reply by: Chris R